1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, a facsimile machine, etc., and a method capable of changing a condition of forming a toner image on an image bearer in accordance with an amount of toner of a reference toner image formed on the image bearer.
2. Discussion of the Background Art
As discussed in Japanese Patent Application Laid Open Nos. 9-211911 and 2006-234862 (JP-H9-211911-A and JP-2006-234862-A, respectively), it is well known in an image formation process that a photoconductive member serving as an image bearer is charged and exposed to an optical image to change a potential of the exposed section and thereby form a latent image thereon. Toner with a prescribed charge is then electrostatically attracted to the latent image, so that a toner image is formed. The toner image is then transferred onto a transfer member.
Further, various transfer processes are employed in such an image formation process. For example, toner images of component colors are temporarily transferred one after another onto the same portion of an intermediate transfer member to be superimposed thereon. The superimposed toner images are then transferred onto a transfer member at once as a color image. Alternatively, toner images of component colors are transferred onto the same transfer member one after another to be superimposed thereon as a color image without using the intermediate transfer member. Yet further alternatively, toner images of component colors are formed on a photoconductive member one after another to be superimposed thereon and are transferred onto a transfer member all at once as a color image without using the intermediate transfer member.
To keep image density constant in such an image formation process, various devices have been employed. For example, as described in the JP-9-211911-A and JP-2006-234862-A, an image forming apparatus is known which forms a reference toner image having a prescribed pixel pattern at a prescribed time, detects a toner attraction amount per unit area in relation to the reference toner image, and adjusts image formation performance based on the detection result. According to this type of image forming apparatus, when an image formation performance changes due to environmental change or as time elapses, a change in the image formation performance is detected based on a change in the toner attraction amount. An image formation condition, such as a charge voltage on an image bearer, a latent image writing power provided to the image bearer, a developing bias, developer toner density, etc., is then adjusted to recover their original performances. As a result, a change in image density caused by the change in the image formation performance can be suppressed.
In theory, the amount of toner attracted to the reference toner image can be expected to remain constant if the charge voltage on an image bearer, the latent image writing power provided to the image bearer, the developing bias, or the developer toner density and the like are also constant.
However, it is known that, in practice, the amount of toner of the reference toner image is affected by the image formed just before formation of the reference toner image. Consequently, adjustment of image formation conditions based solely on the toner amount of the reference toner image, without regard to the condition of the immediately preceding image, runs the risk of destabilizing the density of succeeding toner images formed thereafter.